The invention relates generally to the field of electrical power converters and inverters. More particularly, the invention relates to techniques for preventing failure of motor drive circuitry due to overheating.
Power inverters and converters typically employ power modules to create a desired output current waveform, which is used to power various devices, such as motors and other equipment. The frequency and amplitude of the output current waveform may effect the operation of the device such as by changing the speed or torque of a motor, for example. Some power modules create the desired output current waveform through pulse width modulation, wherein power semiconductor switches such as insulated gate bipolar transistors (IGBTs) are caused to switch rapidly on and off in a particular sequence so as to create an approximately sinusoidal output current waveform. Furthermore, high IGBT switching speeds tend to produce a smoother, more ideal sinusoidal waveform, which may be desirable in some applications. For example, in heating, ventilating, and air conditioning systems a smoother sinusoidal waveform will reduce system noise and vibrations.
Higher IGBT switching speeds may tend, however, to increase the junction temperature of the IGBTs, which may result in more mechanical stress and increased rates of IGBT failure over time. Attempts have been made to reduce IGBT failure by limiting the maximum absolute IGBT junction temperatures. However, these techniques have failed to account for the increased stresses that tend to occur under start-up conditions or low-speed conditions, wherein the IGBTs tend to experience high current at low output frequency.
It may be advantageous, therefore, to provide a system and method of reducing IGBT thermal stress that is particularly effective under start-up conditions and low-speed, high-current conditions. Specifically, it may be advantageous to provide a method of reducing temperature variations of the IGBT junction, i.e. the semiconductor chip itself, and the case, i.e. the package in which the semiconductor chip is contained.